Prosthetic device for the tarsal joint

ABSTRACT

A partial prosthesis for the human tarsal joint has a first member of synthetic plastic material which is to be anchored in the head of the tibia and which has an exposed concavely curved first surface. A second member of metallic material is to be anchored in the talus and has an exposed second surface which is convexly curved on a radius different from that of the first surface, and which engages the first surface with at most line contact.

United States Patent- 1 1 Link 7 1451 Oct. 8, 1974 PROSTHETIC DEVICE FORTHE TARSAL I JOINT v [75] inventor: Helmut D. Link, Hamburg,

Germany [73] Assignee: Waldemar Link, Hamburg,

Germany [22] Filed: June 7,1973

[21] Appl. No.: 367,975

[30] Foreign Application Priority Data July 22, 1972 Germany 2236141[52] U.S. Cl. 3/1, 128/92 C [51] Int. Cl. A6lf 1/24 [58] Field ofSearch..... 3/1; 128/92 C, 92 CA, 92 R [56] References Cited UNITEDSTATES PATENTS 3,140,712 7/1964 Hunter 128/92 C 3,521,302 7/1970Mueller.;... 3/1 3,698,017 10/1972 Scales et a1... 3/1 2/1973 Link 3/1 41973 Averilletal. ..3/1

' FOREIGN PATENTS OR APPLICATIONS 7/1971 Germany 128/92 C OTHERPUBLICATIONS 1.

M.G.l-l. Femoral Condyle Replacements, Vitallium Surgical Appliances(Catalog) Austenal'Company,

1964, page 62.

Primary Examiner-Richard A. Gaudet Assistant Examiner-Ronald L. FrinksAttorney, Agent, or Firm-Michael S. Striker vexly curved on a radiusdifferent from that of the first surface, and which engages the firstsurface with at most line contact.

3 Claims, 5 Drawing Figures PROSTIIETIC DEVICE FOR THE TARSAL JOINTBACKGROUND OF' THE INVENTION The present invention relates to aprosthetic device it was-always thought that it would be impossible toreplace the tarsal joint by a prosthetic device of conventional type,because the approximately roller-shaped talus bone is relatively smalland it is, for instance, impossible to anchor in it a prosthesisutilizing a long shaft which would have to be anchored in the talus bonein order to provide for a proper securement.

Because of the difficulties in employing a prosthesis, an alternativehas been used, namely to stiffen the tarsal joint and make it therebyimmovable, in order to reduce or eliminate the constant pain experiencedunder the aforementioned conditions. This, however, is also not fullysatisfactory for the very reason that the stiffening operation involvesmaking the tarsal joint immovable.

SUMMARY OF THE INVENTION It is, accordingly, a general object of thepresent invention to overcome the disadvantages of the prior art.

More particularly, it is an object of the present invention to provide apartial prosthesis for the human tarsal joint.

An additional object of the invention is to provide such a partialprosthesis for the human tarsal joint which permits the surgeon toretain the form of the bones of the joint largely unchanged.

In keeping with these objects, and with others which will becomeapparent hereafter, one feature of the invention resides in a partialprosthesis for the human tarsal joint which, briefly stated, comprises afirst member of synthetic plastic material which is adapted to beanchored in the head of the tibia, this first member having an exposedconcavely curved first surface. A second member of the prosthesis is ofmetallic material and adapted to be anchored in the talus, and thesecond member has an exposed second surface which is convexly curved ona radius different from that of the first surface and which engages thelatter with at most line contact. I

With this construction, a simple arrangement is .pro-- vided forpermitting relative movement of the two I members and the bones in whichthey are anchored, re-

substance to a very-significant extent, so that even if the operationduring which the prosthesis is inserted should finally be found tohavebeen unsuccessful, it is still possible'subsequently to stiffen thetarsal joint in the previous manner, which would not be possible ifduring the anchoring of the first member in the tibia a large portion ofthe substance of the tibia bone were destroyed for anchoring purposes.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING Hg. 1 illustrates the skeleton of ahuman foot with the partial prosthesis according to the presentinvention being inserted; I

FIG. 2 is a perspective view illustrating a ber of the partialprosthesis;

FIG. 3 is a top view of the second member of the partial prosthesiswhich cooperates with the first member shown in FIG. 2;

FIG. 4 is a bottom view of the member shown in FIG.

3; and

FIG. 5 is a side view of the member shown in FIGS. 3 and 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing,and firstly to FIG. 1, it is emphasized that the skeleton of the humanfoot shown there has'been provided to illustrate where the partialprosthesis according to the present invention is located. The uppertarsal joint, which is to be replaced by the partial prosthesisaccording to the present invention, is clearly visible in FIG. 1 andwill be seen to be located between the head of the tibia bone 1 and theapproximately roller-shaped relatively small talus bone 2. A comparisonof FIGS. 1 and 2 will show that one of the members of the partialprosthesis is substantially first memblock-shaped, being identified withreference numeral convexly curved part-cylindrical surface 6 whoseradius of curvature is substantially smaller than the radius ofcurvature of the surface 5. Thus, when the surfaces 5 and-6 engage one.another they are inline or point contact, but not in surface-to-surfacearea contact. The reduction of frictional resistance which results fromthis, is clearly evident and requires no detailed discussion. In theexample, it is pointedou't that the radius of curvature of the surface 5may, for instance, be 31 mm., whereas the radius of curvature of thesurface 6 may, for instance, be 25 mm., although both of these figuresshould be understood to be exemplary only.

The member 3 is of synthetic plastic material, and high-molecularpolyethylene has been found to be particularly advantageous for thispurpose. The member 4 is of metallic material and its surface 6 ispolished to a high degree of smoothness.

The sides or surfaces of the members 3 and 4 which face away fromsurfaces 5 and 6, respectively, that is the support surfaces whichcontact the respective bones, are provided with anchoring projections 7and 8, respectively, which extend outwardly and conically divergeoutwardly, so that grooves or depressions 9 are formed between them.which are particularly advantageous for anchoring in the respectivebone, using a bone cement which is well known to those skilled in thisfield.

The talus bone 2 is so small that the member 4 which is to be anchoredin it cannot be provided with an elongated shaft or projection to extendinto the talus bone, that is to be driven into the bone 2. Because ofthis, the invention provides that the member 4 is produced in variousdifferent sizes of which the one to be used is so selected--dependingupon the particular anatomical circumstances of a given case--that it isjust large enough to be able to rest with its margins 10 on the hardportions of the bone and not to sink into the talus bone 2. Inaccordance with the invention, it is advantageous under thesecircumstances that the projections 8 of the second member 4 be providedonly in the center portion thereof, as shown in FIG. 4.

The second member 4, which is plate-shaped as mentioned earlier, isapproximately trapezoidal in outline as shown in FIG. 3. The purpose ofthis is to take into account the fact that usually the talus bone isslightly wider at the front side of the foot than at the rear side, sothat the particular configuration chosen for the member 4 can fullyutilize the supporting surface of the talus bone.

Bones have, generally speaking, a hard outer skin, the cortikalis, and asoft inner spongy structure, the spongiosa. That being the case, theimplantation of the projections of the second member 4 is advantageouslycarried out by forming a hole in the outer hard layer of the talus bone,removing (e.g., by drilling) all or most of the spongiosa, and fillingthe resulting cavity with a bone cement which, when hardened, willretain the projections of the member 4. The member 4 will then besupported on a relatively long hard outer surface layer of the talus.

When the prosthesis according to the present invention is used foradults, then a size of the first member 3 of approximately 34 X 40 mm.is usually sufficient, because if the member 3 should be found to be toolarge during the operation, material can be removed from it inasmuch asit is made of synthetic plastic. This is not possible with the secondmember 4 which is of hard metallic material. It is thereforeadvantageous if the second member 4 is made in several different sizes,as mentioned earlier, for instance four sizes on the order ofapproximately 28-29 mm. up to a largest width of approximately 35-36mmand with a length of the non-paralel edges of approximately 35 mm. Themaximum dimension in direction normal to the general plane of the member4 is advantageously between approximately 4 and 6 mm., and the lengthofthe projections is approximately 4 mm. It will be understood that thesimilar maximum dimension of the first member 3 can be larger, becausethe tibia bone 1 permits greater material removal. For this reason it isalso advantageous, in order to obtain an improved anchoringeffect, ifthe various projections 7 on the first member 3 are of various differentlengths, although that is, of course, not absolutely. necessary.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the type described above.

While the invention has been illustrated and described as embodied in aprosthetic device, it is not intended to be limited to the detailsshown, since various modifications and structural changes may be madewithout departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand,therefore, such adaptations should and are intended to becomprehendedwithin the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

1. A partial prosthesis for the human tarsal joint, comprising a firstrigid member of synthetic plastic material, substantially block shaped,having an exposed concavely curved first surface of part-cylindricalshape,

and a support surface facing away from said first surface and includinga plurality of discrete trapezoidalshaped projections of different sizesextending over the entire support surface, substantially diverging in adirection away from said support surface; a second rigid member ofmetallic material, substantially plate-shaped and of substantiallytrapezoidal outline, having an exposed convexly curved second surface ofpart cylindrical shape with a radius of curvature different from that ofsaid first surface, and a support surface facing away from said secondsurface and including a plurality of discrete trapezoidal-shapedprojections substantially diverging in a direction away from saidsupport surface provided on only a center portion of said supportsurface; whereby said first rigid member is adapted to be fastened tothe head of the tibia, and said second rigid member is adapted to befastened to the talus so that said first and second members engage eachother in line contact and have limited freedom of lateral displacementalong the line of contact.

2. A prosthesis as defined in claim 1, wherein different ones of saidprojections of said first member are of different lengths. v

3. A prosthesis as defined in claim I, wherein said first member is ofhigh-molecular polyethylene.

1. A partial prosthesis for the human tarsal joint, comprising a firstrigid member of synthetic plastic material, substantially block shaped,having an exposed concavely curved first surface of part-cylindricalshape, and a support surface facing away from said first surface andincluding a plurality of discrete trapezoidal-shaped projections ofdifferent sizes extending over the entire support surface, substantiallydiverging in a direction away from said support surface; a second rigidmember of metallic material, substantially plate-shaped and ofsubstantially trapezoidal outline, having an exposed convexly curvedsecond surface of part cylindrical shape with a radius of curvaturedifferent from that of said first surface, and a support surface facingaway from said second surface and including a plurality of discretetrapezoidal-shaped projections substantially diverging in a directionaway from said support surface provided on only a center portion of saidsupport surface; whereby said first rigid member is adapted to befastened to the head of the tibia, and said second rigid member isadapted to be fastened to the talus so that said first and secondmembers engage each other in line contact and have limited freedom oflateral displacement along the line of contact.
 2. A prosthesis asdefined in claim 1, wherein different ones of said projections of saidfirst member are of different lengths.
 3. A prosthesis as defined inclaim 1, wherein said first member is of high-molecular polyethylene.